Description of Related Art
Gas turbine engines include one or more rotor shafts supported by bearings which, in turn, are supported by annular frames. The frame includes an annular casing spaced radially outwardly from an annular hub, with a plurality of circumferentially spaced apart struts extending therebetween. The struts may be integrally formed with the casing and hub in a common casting, for example, or may be suitably bolted thereto. In either configuration, the overall frame must have suitable structural rigidity for supporting the rotor shaft to minimize deflections thereof during operation.
These structural engine frames are usually required to transmit loads from the internal rotor bearing support, typically the hub, across the engine flowpath, by means of the equi-spaced struts, to a flange mounted on the case. In order to minimize rotor blade tip clearances and maximize engine performance, deflections of the rotor relative to the static structure must be minimized which may be accomplished by incorporating a sufficiently stiff frame. Frame stiffness is also a very significant factor for controlling rotor dynamics. A stiff support for the rotor will raise rotor natural frequencies above the operating range of the engine, thus preventing undesirable levels of resonances in the engine operating range.
Because the bearing load is transferred into the case at local points, namely the strut ends, the design of the case is important to the overall frame stiffness. Bending can occur in relatively thin annular case sections due to these point loads thereby introducing unwanted flexibility in the engine frame design. However, in order to minimize engine weight and improve aircraft fuel efficiency and cost, it is desirable to maintain the exterior casing at a minimum thickness to the extent that little bending stiffness is offered by the casing itself.
One example of the prior art solution to this problem is shown in U.S. Pat. No. 5,076,049, entitled "Pretensioned Frame" and provides a polygonal exterior casing. The strut end loads are transmitted through the case in direct tension and the case, while still relatively thin, is loaded in tension, rather than bending, and frame is significantly stiffer than previous turbine frame designs. One drawback of this design is that the polygonal case is subject to very high bending stresses because the internal pressure is high during operation and the pressure differential across the case is great. Internal pressure attempts to bend the polygonal panels back to a circular shape and therefore is not suitable in high pressure applications, such as a high pressure turbine exit frame.
Another example of a prior art solution to this problem is shown in U.S. Pat. No. 3,403,889, entitled "Frame Assembly Having Low Thermal Stress" and provides circular rings fabricated on the case. A similar type of design is used on turbine mid-frame of the General Electric CF6-50 aircraft gas turbine engine as shown and disclosed on pages 495-498 and FIG. 25-35 of "Aircraft Gas Turbine Technology, Second Edition" by Irwin E. Treager. These design adds significant I-section ring support to the case thereby counteracting the bending caused by the strut end loads. The increased stiffness afforded by the ring reinforced case improves frame overall stiffness but is still structurally inefficient, in the sense that transmission of loads through bending requires more material for a given stiffness than would be required to transmit loads in direct tension. An advantage of this design is that it can accommodate significant internal pressure, since the casing skin is circular. A disadvantage of this design is that the circumferentially continuous radial height of these rings produces undesirable high thermal stress levels because of the large temperature differentials across the outer casing. These rings are radially constrained and the higher the ring the greater the stress as well as the greater the reinforcing effect on the case by the rings which makes the frame stiffer.
Polygonal stiffening rings have been used on the turbine frame of the General Electric LM6000 marine gas turbine engine. The rings have a polygonal radially outer surface or perimeter and a constant axial thickness. It does not provide a direct tension load path but rather a circumferentially curved load path and is subject to the thermal stress problems as discussed earlier. The structural inefficiency of this prior art design results from the fact that the centroids of the polygonal ring cross-sections do not subtend a straight line from strut-end to strut end. In addition, the cross-sectional area of these same sections is not constant. The result of this is bending in the polygonal stiffening rings and a non-optimum stress distribution An "ideal" design is a "rope", i.e., a member with a constant cross-section & forming a straight line between load points, carrying tension stress only but as with many ideal designs the realities of the harsh engine environment and other design considerations prevent the use of such a "rope" stiffener for the frame to provide a circumferentially linear load path between struts.
A significant drawback of the prior art designs, particularly as applied to hot section applications, is the severe thermal gradient which will develop between the hot case, exposed to engine cycle air on the inner diameter (ID), and relatively cool stiffener rings, exposed to under-cowl air in operation. These gradients cause thermal stresses, as discussed earlier, that lead to cracking of such cases, and sometimes require active heating of the reinforcing rings to prevent such distress. Heating takes away power from the engine and therefore lowers the engine's fuel efficiency. Furthermore, the weight of the associated plumbing and hardware to heat the rings is another disadvantage of such designs.
Accordingly, it is desirable to have polygonal turbine frame stiffening rails that carry substantially only tension stress and very low thermal stresses. It also desirable to have a turbine frame constructed of thin annular casings and radial struts yet which still provide suitable rigidity and structural integrity of the turbine frame for carrying both compression and tension loads through the struts without undesirable deflections of the hub which would affect the proper positioning of the rotor shaft supported thereby.